Combine starter, warm water and bread flour in a large bowl, let stand for 8-10 hours.
Stir in 1 cup of whole wheat flour, followed by the salt and remaining whole wheat flour (and the all-purpose if needed), until the dough starts to come away from the side of the bowl.
Turn out onto a floured surface and knead about 10 minutes.
Place the dough into a greased bowl, turning to coat.
Cover bowl and allow to rise until doubled, about 2-3 hours.
De-gas the dough gently and shape into a round.
Place dough smooth-side down into a banneton or floured tea-towel lined bowl.
Cover and let rise 1 1/2 hours.
Preheat oven to 425F.
Turn dough out of the banneton or bowl onto a greased baking sheet.
Slash dough 2-3 times, bake for 35-40 minutes, until loaf sounds hollow on the bottom when tapped.
Cool completely on a wire rack before slicing.